1. Field of Invention
This invention is concerned with a radio frequency (RF) signal transmission system, and more particularly with such a system which utilizes heterodyne techniques.
2. Description of the Prior Art
Basic RF communications systems consist of transmitters and receivers. Commonly, heterodyne techniques are used to up-convert the baseband information frequency to the transmitter carrier frequency. Conversely, at the receiver, a down-converter is used to extract the baseband signal from the received carrier frequency. To control the baseband frequency so that it falls within the IF (intermediate frequency) amplifier bandwidth at the receiver end, the local oscillators used as the heterodyne frequency sources for the respective up-converter and down-converter must be accurately controlled. If the baseband frequency must be reproduced at the receiver within a specific error range (.DELTA.F), the two local oscillators must be within .DELTA.F of each other. If the baseband frequency must be preserved exactly, the frequencies of the two oscillators must be identical. However, in most applications, the communications process can usually be adequately served with oscillators of crystal-control stability or of even lesser stability.
For a short-range communications link, especially one involved in transmission of wideband digitally coded data, it may be necessary to preserve the baseband frequency closely because of band allocations for the various modulation elements of a complex baseband signal. Expensive crystal controlled frequency synthesizer type local oscillators would be required, should conventional communications link techniques be employed. Where the character of the coded data is such that phase coherence is required for simplified decoding at the receiver, complicated phaselock and clock regeneration circuits would be required.
It is also known to have a system involving an interrogator and a transceiver. The interrogator sends a radio frequency signal to the transceiver which modulates the signal with some code and retransmits the thus modulated signal back to the interrogator which uses its radio frequency signal to demodulate the modulated return signal.